54 
ELEMENTS OF GEOLOGY. 
Fig. 18. Fig. 19. 
Cyrena oiovata, Sow.; fossil. Hants. Cyrena (Corbicella) Jluminalis, 
Moll.; fossil. Grays, Essex. 
altogether extinct and those without shells, amount to 446 
in number, of which the terrestrial and fresh-water genera 
scarcely form more than a fifth.* 
Almost all bivalve shells, or those of acephalous mollusca, 
are marine, about sixteen only out of 140 genera being fresh¬ 
water. Among these last, the four most common forms, both 
Fig, 23 . recent and fossil, are Cyclas, Cyrena^ 
UniOy and Anodonta (see figures); the 
two first and two last of which are so 
nearly allied as to pass into each other. 
Lamarck divided the bivalve mol¬ 
lusca into the Dimyary^ or those having 
two large muscular impressions in each 
valve, as a ^ in the Cyclas, Fig. 18, and 
Unio, Fig. 22, and the Monomyary^ such 
as the oyster and scallop, in which there 
is only one of these impressions, as is 
seen in Fig. 23. Now, as none of these 
last, or the unimuscular bivalves, are 
fresh-water,f we may at once presume a deposit containing 
any of them to be marine. 
* See Woodward’s Manual of Mollusca, 1856. 
t The fresh-water Mulleria, when young, forms a single exception to the 
rule, as it then has two muscular impressions, but it has only one in the adult 
state. 
Gryphcea incurva, Sower.; 
(G. arcuata, Lam.) upper 
valve. Lias. 
